We tested on our '72 and '76 Trans Ams. Because both cars are driven regularly, idle quality and low-speed street manners receive as much attention as full-throttle performance. And even though 455ci engines power both, the two combinations are very different. See a full rundown of the specifications in the accompanying chart.
Vacuum advance increases spark lead to improve engine efficiency in light-load conditions. When set correctly, any engine should benefit from it.
Prior to beginning the experiment, we determined the amounts of initial and total spark lead that the engines seem to perform best with. Using professional equipment, we carefully calibrated each distributor's centrifugal advance curve. Over the course of several weeks and several hundred miles, we recorded engine vacuum levels and noted throttle response and overall performance in every driving condition without any vacuum advance whatsoever.
Past testing indicates that Pontiac engines typically respond favorably to 12-16 degrees of vacuum advance, bringing total spark lead (initial plus centrifugal plus vacuum advance) into the 45-50 degree range at part throttle. Although some engines may tolerate even greater amounts without detonating, the effects appear to diminish with spark lead much beyond that.
After selecting vacuum-advance units that best complement our engines' operating characteristics, we installed them and connected them to ported vacuum. We immediately noticed a dramatic increase in part-throttle response from each engine with roughly 15 degrees of vacuum advance, especially at sustained cruising speeds. Vacuum levels in those conditions jumped 1 to 1.5 inches, indicating improved engine efficiency. We also set both carburetors leaner, which certainly contributed to the fuel economy increase of nearly 2 mpg we saw from both vehicles.
The most consistent source for ported vacuum is this one located on the carburetor main body. | 
It receives its vacuum signal from this slot found in the primary bore, which is covered by the blades at idle. Once acceleration begins, the slot is exposed to manifold vacuum, and vacuum above and below the throttle blades equalizes. | 
Stock-replacement vacuum-advance units, like these from Borg-Warner (or equivalent), are available from most any parts store. These particular units were chosen because they had the desired specifications to complement our combinations. |
Manifold-Vacuum Advance On The '76 T/A
As strong as both Firebirds felt at part-throttle, we knew that manifold vacuum advance oftentimes improved idle quality. The '76 Trans Am's large flat-tappet camshaft idled rough and barely produced 8 inches of vacuum at 1,000 rpm. So we chose a stock-replacement vacuum-advance unit that operated with the least amount of vacuum hoping it would fully deploy at idle. However, there was little idle quality improvement. It wasn't until we revved the engine and increased vacuum that we saw a 250-rpm increase in idle speed.
After resetting the idle speed and readjusting the idle-mixture screws, the initial road test proved very disappointing. Every time the clutch was engaged, engine vacuum dropped and the unit retracted, causing the engine to stumble severely at take-off. Once underway and vacuum levels rebounded, the engine surged violently while coasting or decelerating. It became apparent that the stock replacement unit wasn't correct for this manual-trans-backed engine; an adjustable aftermarket unit was likely required.
The available advance of the aftermarket unit was limited to 14 degrees, and the vacuum-activation point was lowered considerably. Once installed and connected to manifold vacuum, idle speed increased as soon as the engine fired. After resetting the idle speed and readjusting the mixture screws, vacuum increased about 3 inches. The large camshaft seemed much tamer, and idle quality was smooth and stable. Additionally, the exhaust note gained a pleasant, finely tuned sound.
Subsequent road tests proved that the engine was largely unaffected by clutch engagement, and acceleration was extremely smooth and effortless. Surging, however, was still present, although not nearly as severe. The most logical corrective action was to further reduce vacuum advance. After a number of different static initial and vacuum-advance settings, we ultimately arrived at 14 and 12 degrees, respectively. This limited surging to extreme driving conditions and reduced idle vacuum by 0.5 inches, but there was no detectible loss in idle quality or low-speed street manners.
We needed Crane's adjustable vacuum advance kit in one instance. Offering a wide range of vacuum and advance settings, it seems most beneficial when tuning radical combinations. Installation details were covered in "Opportune Timing," which appeared in the Jan. '04 issue of HPP.
Manifold-Vacuum Advance On The '72 T/A
After moving the '72 Trans Am's vacuum-advance unit from ported vacuum to manifold vacuum, idle speed immediately increased about 200 rpm. After the preliminary adjustments, idle quality noticeably improved, and vacuum jumped roughly 1.5 inches. The load generated by placing the automatic transmission into gear seemed to affect the engine less, another indication that it was operating more efficiently on the available fuel mixture.
A definite improvement on idle, initial take-off, and light-throttle acceleration characterized the overall effect of manifold-vacuum advance over ported. We did not, however, expect or notice any real difference in part-throttle response over ported vacuum. What we did discover, though, was a smoother transition when reapplying the throttle after coasting at speed, likely caused by the spark lead that manifold vacuum advance maintains while coasting and decelerating.